The present invention relates to power cables of the mass-impregnated type, i.e., metal sheathed cables insulated with paper which is impregnated with a viscous compound (also called Solid Type Paper Insulated Cables), and is especially related to such cables designed for high voltage direct current (HVDC) transmission, underground as well as submarine in shallow waters.
Actually the mass-impregnated cable is the oldest type of high voltage cable. Already at the turn of the century, 85 years ago, such cables were in use for voltages up to 10 kV. Later the voltages increased thanks to improvements of the insulating materials as well as to development of the required production processes.
However, experience showed that the normal mass-impregnated cable has one important draw-back. In steep slopes of the cable route, the impregnant has a tendency to migrate from a higher level of the route to a lower one, with the result that the insulation of the cable at the higher level would get voids which could result in ionization and break down of the insulation. Such migration of the impregnant is accellerated when the temperature of the cable varies, for instance due to load variations. When the temperature increases, the impregnating compound will expand and press out the diameter of the metal sheath. When the cable is cooled again due to reduction or switching off of the load, the impregnant will contract. However, since the metal sheath material used, such as lead, is a rather soft and plastic material the expanded diameter of the sheath will mainly remain, and there will be a deficit of impregnating compound also at the lower level of the cable route giving room for additional migration from the higher level. The next time the cable is loaded, new expansion takes place and so on. Since the viscosity of the impregnating compound changes strongly with the temperature (reducing with increasing temperature), an increase of the maximum temperature will also contribute to accelleration of the migration. For these reactions limitations had to be introduced regarding the use and application of the mass-impregnated cable type. The rated AC voltage was limited to 60 kV and the maximum operating temperature was limited to 40.degree.-70.degree. C. depending on the rated voltage.
When higher voltages and higher transmission capacities were demanded, other cable types were developed, based on pressurization of the insulation (pressure higher than the atmospheric pressure, the so called pressure cables). Examples of such cables are the oil-filled types, the gas-pressure cables, etc. In such cables provisions are made to keep the pressure of the insulation at a minimum positive design pressure (which may be just above the atmospheric pressure or a high pressure of some 15 bar) in the entire cable length under all load conditions. These cables can be used for considerably higher voltages and temperatures than the normal mass-impregnated cables. For instance, selfcontained oil-filled cables have been designed for voltages up to 1100 kv AC and maximum temperatures of 85.degree.-90.degree. C. As a consequence, the mass-impregnated cable is generally not used for AC voltages above 20-30 kV.
Pressure cables are designed with continuous duct(s) in the cable communicating with degasified oil (oil-filled cables) or gas (gas-pressure cables) stored in tanks at certain places along the cable route. The pressure may be obtained in two ways: either by means of static pressure tanks or by means of pumping plants. Especially in connection with long HVDC submarine oil-filled cables (which are more used than gas-pressure cables--for reasons which are not discussed here) large quantities of oil are needed at the terminals of the cables to keep the pressure at the prescribed minimum level under all load conditions. Due to the dynamic pressures which are generated as the oil is flowing along the duct(s) to or from the oil tanks during heating and cooling respectively, the pressure at the terminals must be high. Since the pressure at the terminals must be limited (for mechanical reasons), this will contribute to limiting the length of an oil-filled cable. Another draw-back with the oil-filled cable type used as submarine cable is connected with the risk of failure of the cable, for instance due to anchoring, fishing tackle etc. If a failure occurs, the fluid impregnating oil will flow out of the cable at the failure spot, and since several days--some times several weeks--may pass before the failure can be repaired, large additional quantities of degasified oil are needed at the terminals to refill the cable, and there is always the risk that water will penetrate into and/or along the cable duct damaging a great part of the cable insulation and making the repair very difficult. However, as far as the insulating system is concerned, the oil-filled cable type may be designed for DC voltages up to at least 1000 kV when the cable route is otherwise adequate for the oil-filled cable type.
Gradually the market for mass-impregnated cables is also vanishing for the lower AC voltages up to 20-30 kV due to the development of plastic insulated cables. However, in connection with the introduction of high voltage direct current (HVDC) transmission of the mass-impregnated cable has met with a new era, especially for submarine transmission. There are several reasons for this:
The cable type may be used as submarine cables in practically unlimited lengths (only limited by the voltage drop) PA1 The cable type may be designed for reasonably high DC voltages (the maximum voltage for cables in actual service today is slightly below 300 kV). PA1 The cable type is simple and robust compared with the pressure cables (simple cable design, simple repair if a failure should occur, the water will penetrate only a few meters along the cable). No large stores of oil or complicated pumping plants are needed at the end terminal(s); only one small static pressure tank filled with a compound compatible with the cable's impregnant is needed at each of the end-terminals mainly to keep the pressure of the end-terminal's insulation above the atmospheric pressure. Thanks to the viscous impregnant which acts as a lubricant of the paper-tapes the insulation endures rough mechanical handling of the cable.
However, the risk of migration of the impregnant in steep parts of the cable route causes a limitation as to the voltage for which it may be designed, also in the case of HVDC cables. The purpose of this invention is to overcome this draw-back by designing and manufacturing a mass-impregnated cable in such a way that it acts as a pressure cable under all load conditions.
It has been demonstrated by tests that the pressure of the insulation in a mass-impregnated submarine cable depends on the sea depth. At certain depths depending on the cable design, ambient temperature, load conditions etc. the pressure will always be higher than the atmospheric pressure and the pressure increases with increasing sea depth. This is due to the outer water pressure. At such depths the cable is operating as a pressure cable under all actual load conditions. Since the pressure of the insulation increases with increasing sea depths, no migration of the impregnant can take place in deep water from an upper level of the route to a lower one, regardless of the steepness of the route. The weak parts of a submarine mass-impregnated cable are therefore located to the shallow waters and the parts on land since in these parts of the cable route there may be a risk of migration of the impregnant of the cable insulation. "Shallow-waters" in this sense are waters with sea depths less than 50-200 m depending on cable design, ambient temperature, load conditions, etc.
It has been suggested that the mass-impregnated cable in the shallow parts of a route be replaced with oil-filled cable. However, if the shallow part of the route is very long (in some cases it may be several hundred km), an oil-filled cable cannot be used.
To overcome some of the said problems it has also been suggested that the mass-impregnated cable be provided with a pressure-body consisting of a number of elastic metal tapes applied edge to edge directly on the lead sheath, Swedish Pat. No. 115089.
A drawback of this design is that the tapes, which are applied in one layer on the lead sheath, must be applied with a relatively long lay-length, involving a poor utilization of the material since the tension of the tapes will have to be relatively high to obtain sufficient pressure of the tapes against the lead sheath during operation of the cable and therefore the tapes must be relatively thick in order not to exceed the yield strength of the tapes.
Further the application of the metal tapes edge to edge is hardly practical apart from the fact that when the tapes are laid edge to edge during application, they will not function as intended when the cable is colder than the temperature in the factory during application. Certainly, in a very short cable this drawback can be overcome by pressing impregnating mass into the cable after the application of the tapes as suggested in the patent. But from a practical point of view, this is not possible in a long cable since the time for such an operation would be far too long and uneconomical.
Another drawback with this design is the application of the tapes directly on the lead sheath. Due to the high tension of the tapes, the edges of the metal tapes will have a tendency to cause injury to the lead sheath, since lead is a very soft material. Such injury is accelerated when the cable is hot, partly because the lead gets softer, partly since the tension of the tapes will increase due to the expansion of the cable. Because of the cables expansion there will be a gap between the tapes even if they are applied with edge to edge during the manufacture and due to the internal pressure the soft lead will have a tendency to be pressed into these gaps, and in the long run this may result in cracks of the lead sheath at the edges of the tapes because of the relatively great thickness of the metal tapes.